In a thermocouple, a voltage is developed when different temperatures are applied to the opposite ends thereof. Thermoelectric power generation makes use of the voltage as electric energy.
Thermal power generation has attracted much attention lately as an effective means of converting thermal energy directly into electric energy including the utilization of waste heat.
In particular, a thermoelectric power generation unit used for generation of thermoelectric power has attracted much attention because of its potential for application to micro-sized portable electronic equipment, for example, a wrist watch, since it has advantages over other types of power generators in that it is more adaptable to miniaturization owing to its simple structure composed of a plurality of thermoelectric power generation elements, namely, thermocouples, connected together in series, and further it will pose no problem of battery depletion or leakage of the electrolyte in a battery unlike the case of a redox battery.
FIG. 46 is a perspective view showing an example of a structure of a conventional thermoelectric power generation unit. This thermoelectric power generation unit has a laminated structure as a whole, and comprises a plurality of thermocouples 100 consisting of P type thermoelectric material 101 and N type thermoelectric material 102; a large number of the thermocouples serving as thermoelectric power generation elements are disposed in a predetermined manner and electrically connected in series.
A hot junction 104 and a cold junction 105 of respective thermocouples 100 are disposed on the upper surface and the underside surface, respectively, of the thermoelectric power generation unit with the laminated structure so that power is generated by a difference in temperature between both surfaces.
Thermoelectric power is generated by utilizing the so called "Seebeck Effect" wherein if two dissimilar metals are connected together at opposite ends, and a difference in temperature is maintained between two junctions, a thermoelectric e. m. f. is generated between the two junctions.
Such a thermoelectric power generation unit for generation of thermoelectric power as described above is normally manufactured by the following method.
Firstly, pulverized alloy particles are sintered to form a block of process material; P type and N type thermoelectric semiconductor materials in block form are prepared by what is called the "sintering method".
Then, each block thus formed of respective thermoelectric materials is cut and broken into chips in the shape of a rectangular parallelepiped by a dicing saw or the like. The chips with the rectangular parallelepiped shape are arranged in a matrix fashion as shown in FIG. 46 such that a P type thermoelectric material 101 and an N type thermoelectric material 102 are alternately disposed.
Subsequently, a thermoelectric power generation unit comprising a plurality of thermocouples connected in series is fabricated by connecting the opposite ends (at the hot junction 104 and the cold junction 105, respectively) of adjacent chips with a conducting member such as a metal plate or the like, such connection being made mainly by soldering.
Typically, the conventional thermoelectric power generation unit fabricated by the method stated above is of a shape several tens cm square or larger and comprises scores of thermocouples.
The output of a conventional thermocouple composed of a Bi--Te based material, which is believed to have the highest performance among all the thermoelectric materials in practical use, is on the order of 400 .mu.V/.degree.C. per thermocouple.
A wrist watch, representative of portable electronic equipment, is normally used in an environment at around room temperature, and consequently, a large difference in temperature between any parts inside a wrist watch can not be anticipated; the magnitude of temperature difference therein will be around 2.degree. C. at most.
Due to such small difference in temperature inside a wrist watch, as many as 2,000 thermocouples composed by a Bi--Te based material will be required to generate a voltage needed to drive a wrist watch, namely 1.5V or higher.
This may not pose any problem if a thermoelectric power generation unit can be enlarged in dimensions. However, it is very hard to integrate as many as 2000 thermocouples in a space 1 cm square, substantially equivalent to the size of a button sized cell. The size of a thermoelectric power generation unit is important particularly when it is to be used as a power supply source of a wrist watch and other micro-sized electronic equipment.
Simply stated, the miniaturization of thermoelectric power generation units may be achieved only if a sintered body of thermoelectric materials can be cut into minute pieces by mechanical working as stated in the foregoing.
However, working on micro-sized elements has naturally its limitations and owing to the very fragile nature of most thermoelectric materials, caution needs to be exercised not only to a cutting process but also to a handling thereof after the cutting process, inevitably resulting in a lower production yield.
The minimum size of a material that can be handled in the conventional method of manufacturing by mechanical working is considered to be about 1 mm square, and even if a thermoelectric power generation unit is packaged into a space 1 cm square, the maximum number of thermoelectric power generation elements, namely thermocouples, incorporated therein is estimated at around 50 couples only.
There is another conceivable method of manufacturing a thermoelectric power generation unit wherein a thermoelectric material in a thin film form is formed by the vacuum deposition process, small-sized thermocouples are fabricated by preparing the micro-sized thermoelectric material in a thin film form through the etching process, and the thermocouples thus fabricated are connected in series, completing a thermoelectric power generation unit. Certainly, manufacturing small-sized thermocouples will become easier with this method.
However, the film formed by the vacuum deposition process is around 1 .mu.m in thickness, which is too thin to compose thermocouples for use as thermoelectric power generation elements, and when as many as 2000 of such thermocouples are in place, the internal impedance thereof will become too high, posing the risk of the thermocouples being unable to output a current strength required of the thermoelectric power generation elements.
Therefore, thermocouples fabricated with films formed by the vacuum deposition process are unsuitable for use as the thermoelectric power generation elements.
Still, a further process proposed for forming the thermoelectric power generation elements is a so-called thick film process wherein a Bi--Te based alloy in a pasty state is applied and then sintered to form a film significantly thicker than the thin film formed by the vacuum deposition process.
The method of manufacturing a thermoelectric power generation element by the thick film process is disclosed, for example, in Japanese Patent Laid Open, JP. A S63-70462.
By the thick film process disclosed in the JP. A S63-70462 fine patterning is feasible by means of screen printing, and furthermore, a film as thick as 10 .mu.m or more can be formed. For this reason, the thick film process is more suitable for forming thermoelectric power generation elements with low internal impedance than the method of forming a thin film by the vacuum deposition process.
However, the thick film process is very complex since it requires preprocessing steps comprising: melting first process materials such as Bi, Te, Sb, Se or the like, making an ingot of alloy from molten metals mixed, and crushing the ingot to make paste from pulverized alloy, without allowing the process materials to be just mixed in their raw state and applied.
Furthermore, this process poses risks of the process materials being contaminated by impurities in one of the preprocessing steps of making the pasty material, uneven distribution of chemical composition occurring in the material prepared as above due to failure in making a uniform solid solution, and cracks occurring in the course of sintering.
With this process, patterning by screen printing is possible but high precision fabrication of micro-sized thermoelectric power generation elements is hard to achieve. For the reasons stated in the foregoing the thick film process is not good enough to provide thermoelectric power generation elements having satisfactory characteristics, and therefore, not the most suitable method for manufacturing micro-sized thermoelectric power generation units.
As is clear from the foregoing description, by means of either the conventional mechanical working method or the manufacturing method through etching the film formed by the conventional vacuum deposition process, it is hard to manufacture a thermoelectric power generation unit by integrating a number of thermocouples as the thermoelectric power generation elements in a minuscule region, and a manufacturing method for a micro-sized thermoelectric power generation unit with a sufficient output capacity has not been available.
On the other hand, the thick film process as stated above is not fully satisfactory in respect of the complex nature of the process and the product characteristics.
Therefore, it is an object of the present invention to provide a method of manufacturing with high precision and with ease a micro-sized thermoelectric power generation unit having a sufficient output capacity as a generator and with a capability of high precision patterning as well, solving problems encountered by the conventional methods for manufacturing a thermoelectric power generation unit.